GB779474A - Method of preparing silicon nitride articles by slip casting - Google Patents

Abstract

Articles are cast from silicon or silicon nitride or a mixture of both by a method in which a suspension of the powdered substance is formed in a suitable medium. e.g. water, a peptizing agent is added to adjust the pH of the slip to between 7,5 and 10, the slip is poured into a mould and the cast article is removed therefrom. The article is then nitrided by heating it in nitrogen, preferably at a temperature of 1175 DEG C. followed by 1300 DEG C. Silicon must be comminuted to a particle size of not more than 5 microns, 3 microns being a preferable maximum, and should be commercially pure (97 per cent), and the specific gravity of the aqueous suspension should be as high as possible, i.e. the silicon content should be 60 per cent to 70 per cent by weight. The peptizing agent renders the slip sufficiently fluid for pouring and may be added either before or after the powder is introduced into the suspension medium, the quantity ranging from 0,01 per cent to 1 per cent by weight of the medium. Suitable peptizing agents to give the above pH range are monoethanolamine, sodium hexametaphosphate and sodium pyrophosphate. The particle size for silicon nitride is 100-325 mesh. For bonding purposes a minimum of 1 per cent silicon, of 5 or less micron particle size, should be added to the suspension. Three examples, specifying proportions of ingredients by weight, are given of making castings from slip of silicon or silicon metal.